The present invention relates to tuning circuits, and more particularly to an improved varactor circuit and method of making a varactor circuit for tuning a dielectric resonator oscillator (DRO) that facilitates production of DROs and that compensates for non-linearities in the varactor.
Digital microwave radios, and other high frequency communication and/or data processing devices, may include devices with electronically tuned frequency sources. Such frequency tuning sources may include a dielectric resonator and a varactor circuit to form a stable and adjustable frequency source. With reference to FIG. 1, a tuning source 10 of the prior art may include a transmission line 12 connected to an oscillator (not shown) and a varactor circuit 14 coupled to the line 12 through a dielectric resonator 16. The transmission line 12 carries signals that control oscillator frequency. The varactor circuit 14 is separated from the line 12 and includes a varactor 18 (a capacitor that varies according to the voltage applied to its terminals) at one end and an open circuit at an opposing end. The varactor circuit 14 may be on or in a substrate 20 that may also carry the transmission line 12 and the resonator 16.
When the source 10 is operating, the resonator 16 is electromagnetically coupled to the line 12 (shown by magnetic flux line 22) and to the varactor circuit 14 (shown by magnetic flux line 24) so that a change of voltage in the varactor 18 causes a change in the signals carried by the line 12. Thus, the frequency of the oscillator may be changed by changing the voltage provided to the varactor 18. The source 10 functions like a filter in that signals in the line 12 have their characteristics (e.g., frequency band) modified by the effect of the magnetic field of the resonator 16 on the magnetic field of the line 12.
The frequency band and speed of the source 10 are influenced by the physical characteristics and arrangement of the line 12, the resonator 16 and the varactor circuit 14. That is, the size and shape of the circuit 14, the size and shape of the resonator 16, and the distance of the resonator 16 from the line 12 and from the circuit 14 may be selected to provide the source 10 with predetermined characteristics. See, for example, the linear and arcuate varactor circuit patterns illustrated in U.S. Pat. No. 4,835,498 issued May 30, 1989 to Rouger, et al. Numerous arrangements of resonator, varactor circuit and transmission line provide a wide variety of tuning characteristics.
In some applications, such as in a phase lock source (PLS), the electronic tuning is desirably linear. That is, there is a linear relationship between the voltage applied to the varactor and the resulting frequency band. However, a non-linear varactor is typically favored for use in a low noise, narrowband frequency tuning DRO. The non-linear varactor introduces an undesirable, non-linear frequency versus varactor voltage relationship. The pattern of the varactor circuit (e.g., size and shape) may be selected to compensate for this non-linearity, and various techniques for determining the appropriate pattern are known and need not be considered in detail herein. The techniques are typically empirical, rather than analytical and thus the results are more likely achieved through trial and error than prediction.
Nevertheless, when the numerous arrangements of resonator, varactor circuit and transmission line are considered with the various, unpredictable patterns that may be used to compensate for a non-linear varactor, it is apparent that the manufacture of these devices can be expensive because of the plethora of combinations available. A different substrate could be required for each varactor circuit pattern if one pattern were printed on a substrate. The manufacture of separate substrates with different varactor circuit patterns increases unit cost and time of manufacture. To lower cost and to facilitate manufacture of the devices, the number of manufacturing configurations is desirably reduced. Specifically, it is desirable to reduce the number of substrates with different varactor circuit patterns that have to be formed for the desired values of operating characteristics.
In the present invention, one substrate may be used that includes all of the determined varactor circuit patterns, thereby facilitating manufacture of varactor circuits and the devices in which they operate. The determined varactor circuit patterns are divided into elements, some of which are common to all the determined patterns, and others of which are small enough so they do not resonate or otherwise affect the characteristics of the signal in the transmission line. The common pattern is thereafter selectively connected to small elements to form a particular pattern to compensate for the non-linear varactor, and to provide the desired device characteristics.
Accordingly, it is an object of the present invention to provide a novel tuning circuit and method of tuning that obviates the problems of the prior art and facilitates the production of the tuning circuit.
It is a further object of the present invention to provide a novel tuning circuit and method of tuning in which tuning circuit patterns for each predetermined value of an operating characteristic are provided in separate circuit elements that may be selectively connected to form a particular tuning circuit.
It is yet a further object of the present invention to provide a novel tuning circuit and method of tuning in which tuning circuit patterns include separate circuit elements that do not affect tuning when not selectively connected.
It is still a further object of the present invention to provide a novel tuning circuit and method of tuning in which plural tuning circuit elements may be selectively connected to change the width and/or length of the tuning circuit.
It is another object of the present invention to provide a novel tuning circuit and method of tuning in which concentric tuning circuit elements and/or linear tuning circuit elements may be selectively connected to change the tuning frequency of the circuit.
It is yet another object of the present invention to provide a novel method of compensating for a non-linear varactor in a tuning circuit in which separate circuit elements are selectively connected to provide an appropriately shaped varactor circuit.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.